I recently had the chance to speak with a Fokker 50 technician. I asked him if the reverse thrust that the F50 uses is just an inverse pitch on the blades, but he mentioned that there was also some sort of separation that occurred between the propeller shaft and something else I didn't quite understand. Could someone please explain how this works? Thanks in advance

I'm not sure if this is what he meant, but many of the Pratt & Whitney Canada engines utilize a free power turbine design. The hot exhaust gases pass through one turbine that drives the compressor, then through a seperate, non-connected turbine (or series of turbines) that drives the propellor shaft or gearbox. There's no mechanical connection between the power turbine and the compressor shaft; it's only driven by the expanding gases. This doesn't have much to do with reverse pitch though. As you seem to understand, the blade angle just reverses slightly, so with the propellors being driven in the same direction, they're producing thrust in a forwards direction, not rearwards.

Reverse thrust on the F50 is ONLY by changing blade angle of the prop....nothing else is disconnected or whatever.
As Bri2k1 explains, the PW125 has a free power turbine but this is through the whole power range including reverse.
Small trivia: the PW125 exhaust delivers around 250lbs thrust in TO power

Not exactly Alfredo,
The PW125 is a triple spool design where only the 3rd spool powers the propeller.....so basiscally it's a straight twin spool turbo-jet engine with an added turbine and shaft which drives the prop.
Pilots also rarely use reverse as the pitch of blades in ground idle is more than enough to slow the aircraft down ( then you have 2x 12 feet diameter flat discs straight into the airflow ).

The Props on the Rolls Royce Darts are only constannt speed props and no reverse is available but upon landing ground fine picth is selected and as 2enginesonly said, its like having two large metal discs acting as brakes.

Reversebale pitch props are/were not used just on Turboprops but on bigger planes like the bigger Constellation and the DC6 or DC7....I am sure that helped slow those big lumbering giants down quite quickly.

However I have seen Garrett powered Turboprop aircraft actually back up as they threw the props into reverse pitch. In effect, the Fokker 50 you spoke of should have reverse pitch available, Im not sure..So actually they could pull into the gate straight away and then back up sort of like the DC9s do up in DTW at NW....

In the top of the flight deck of the Bristol Britannia there was a removable hatch and when the aircraft was backed using it's own power, the F/E would remove the hatch stand on his seat so he could pop out the top and keep a look out. You had to be careful though and make sure everything in front of the aircraft was secure as the reverse thrust could blow things away.

The Britannia had a free turbine just for the prop and it also had fully reversible props, and on landing the F/E would select reverse idle[called brake dwell] and you very rarely needed much more power as brake dwell was very effective at stopping you. In fact my first landing on a VC-10 I thought they had forgotten to select reverse power as this was far less effective than idle reverse on a Brit. Still four 16ft dia props going into reverse would provide a lot of stopping power.

No it was not a mistake on the Brit the pilots never touched the throttles and as I said even reverse was selected by the F/E.

Now that is what I call a proper aeroplane

Access-air

Not only the big Connies had reverse, many if not all of the 749 Connies had reversible props too, and how could you call the Connie lumbering, the Strat yes , but not the sleek Connie.

I just wasnt sure if the smaller models of the Connie had them. I stand corrected...Yeah I know, the Connie looked like the Concorde compared to the Stratocruiser...LOL.....Wish I had been around to see them.......Oh well...

The original topic of this was reverse thrust on turboprops. At issue here is where the reverse thrust comes from. It is, in all cases, from the blade angle of the propeller. None of the aircraft reverse or deflect the hot stream exhaust gas.

TurboProp engines come in two basic varieties: "Fixed" Shaft and Free Turbine. On the latter, the turbine stage that is connected to the propeller and it's reduction gear box is not directly connected to the shaft of the gas generator turbine. On those engines, the speed of the gas generator determines the thrust available to drive the turbine connected to the prop. The blade angle and air load determine the speed or revolutions of the propeller. Engines of this type include most made by Pratt & Whitney, the GE CT7, and the Rolls Royce Tyne.

The fixed shaft types of engines have a reduction gear off of the same shaft that runs the compressor and turbine stages. Of course, the propeller rpm must be reduced to a much lower value than is efficient for the gas turbine section. These engines run in a very limited speed range. Increased fuel flow is translated into a blade angle change rather than a rpm response. Direct control of the propeller, for Beta or Reverse, allows for low or reverse thrust operation on the ground. Engines of this type include the Allison 501, Garrett TPE 331, and Rolls Royce Dart*

The Dart engine does require an additional comment. Almost all Darts come with a Dowty Rotol propeller system. These do not have reverse blade angles available. They also do not have a separate propeller control. The propeller operates through a range of angles in flight that allow the engine to produce it's range of power from takeoff to idle descent. The engine and propeller will operate in a modest range of RPM in these modes of flight. On the ground, the prop operates at nearly flat pitch which is called Ground Fine. It produces nearly zero thrust. When the engine is operating at around 11000 RPM, the propeller is just on the edge of it's governing and thrust producing range. With a slight power increase, the prop will make enough thrust to get the plane moving on the ground. In the range of 10000-11000, the prop is essentially a flat metal disc that produces drag. The faster the plane goes, the more drag effect is produced. So taxiing at this setting is the easiest to control speed with the brakes.

I think you will find that the R/R tyne was not a true free turbine engine as
it had one turbine to drive the HP compressor and a further set of three turbines to drive both the LP compressor and the propellor.

Now the Bristol Proteus was a true free turbine with a turbine purely for driving the the prop. This caused the delay in the production of the engines original aircraft the Britannia when the prop reduction gear failed so completely off loading the turbine, which oversped and came apart causing the test aircraft to do a forced landing

Quoting 113312 (Reply 9):These engines run in a very limited speed range. Increased fuel flow is translated into a blade angle change rather than a rpm response.

The RPM must respond first, then blade angle changes to correct it. Just like a piston engine with a constant speed propeller. This is the distinction between alpha and beta ranges of control. In alpha the propeller controller governs RPM with blade pitch, in beta it just controls blade pitch (although I'm sure the engine fuel controller has an overspeed governor too).

Quoting 113312 (Reply 9):The blade angle and air load determine the speed or revolutions of the propeller. Engines of this type include most made by Pratt & Whitney, the GE CT7, and the Rolls Royce Tyne.

Even so, in most free turbine applications, propeller RPM is more or less fixed. It might have two settings. The core engine speed varies more, of course.

The term Beta is often used for reverse in both fixed and free turbine turboprops, even though the actual control system may be different between the two types.

The glass isn't half empty, or half full, it's twice as big as it needs to be.

Quoting Jetlagged (Reply 11):The term Beta is often used for reverse in both fixed and free turbine turboprops, even though the actual control system may be different between the two types.

Beta refers to a control regime rather than reverse specifically. In beta mode (usually used on the ground), the power lever directly controls the pitch of the propeller blades, and the engine responds to maintain a minimum prop rpm. In alpha mode (usually used in the air), the power lever controls the power output of the engine, and the prop governor adjusts the propeller blade pitch to maintain prop rpm as set by the condition lever.

Quoting Jetlagged (Reply 11):Even so, in most free turbine applications, propeller RPM is more or less fixed. It might have two settings. The core engine speed varies more, of course.

Hey Jetlagged, I agree with your post until here. If I understand what you're saying maybe it's just backwards. I've flown the Turbo Commander 1000, the YS-11 and the G-159 in the "geared prop cat and several models of the KingAire in the "free turbine "cat and the prop rpm (free turbine) definitely changes with engine rpm (eg. at idle on the ground the prop is lazily spinning but the geared props will stay at a high rpm even at idle.) This was many moons ago but I remember in the TC 1000 after eng start you had to momentarily ease into reverse to remove a grd lock before you could add power to taxi all the time the prop rpm was relatively fixed where as the good old Kingaire started in feather but after start just idle in a low rpm. CC

At low power (start up, ground idle, etc) Np cannot reach the governed RPM, so will be lower than that of course. Engine controls differ in detail as well. I should have stressed I meant at typical in flight power settings. In normal operation Np is governed over a fairly narrow range. In the case of the PW120 series it's simply two set speeds, not variable with the propeller lever. I think the CT7 is similar.

Even "fixed" shaft turbo prop propellers will run at less than governed RPM at idle, for the same reasons, but not as low Np as free shafts, due to the direct mechanical connection between the prop and the engine core.

Using the term "geared" does not differentiate between "fixed" shaft and "free" shaft propellers, since all turboprops need a reduction gear.

Quoting Ralgha (Reply 12):Beta refers to a control regime rather than reverse specifically.

Agreed. Reverse pitch is a part of the beta range. But it's not specific to "fixed" shaft turboprops, which was really my point.

The glass isn't half empty, or half full, it's twice as big as it needs to be.

Quoting Jetlagged (Reply 14):Using the term "geared" does not differentiate between "fixed" shaft and "free" shaft propellers, since all turboprops need a reduction gear.

Ok, that's just the nomenclature I've used over the last 35 yrs to differentiate between a free turbine (PT-6) and the other "fixed" shaft eng. Probably picked it up in one of the schools yrs ago. I don't know about the "2 settings" for the prop speed on the free turbine eng, I certainly don't remember that with the PT-6 and haven't flown the PW120. The last turbo prop I flew was for a short time in '82, the YS-11.Noisy, fixed shaft, water injected R.R.

Quoting Jetlagged (Reply 14):In normal operation Np is governed over a fairly narrow range. In the case of the PW120 series it's simply two set speeds, not variable with the propeller lever. I think the CT7 is similar.

Despite not setting foot on them in ten years (Saab 340's)....

On the Saab 340A (with CT7-5A2's), no set speeds to my knowledge.

During flight the Np was governed between 1200 rpm (min) and 1384 rpm (max), so any speed in between could be set with the condition levers. In reality 'Max' (1384) was used for takeoff/final approach and 1230 for the cruise although in the early days of our Saab operation 1260 was used for cruise.

On the ground, if the bottom governor was enabled, (condition lever to min) the Np was governed between 1000 rpm at Flight Idle to 1200 rpm at max reverse.

I have another question guys. What and why is the condition of a turbo prop when the blades are in the feathered position and are spinning at a low, constant RPM. I usually notice this just as the engines are being started or right after landing while taxiing to the gate and also while in the queue for take off. I assume it saves fuel, but what is this condition called?

Quoting TinPusher007 (Reply 17):What and why is the condition of a turbo prop when the blades are in the feathered position and are spinning at a low, constant RPM. I usually notice this just as the engines are being started or right after landing while taxiing to the gate and also while in the queue for take off.

I also believe this is a safety component built in. I've heard of "automatic feathering," when the engines are shutting down or are at low idle speeds.

Quoting Doug_Or (Reply 18):It burns less gass and on the PW-120 you must be feathered for 1 minute before shutdown (so the engine cools down)

I'm not going to say this is wrong, because I fly the PW118 not the 120, but I am skeptical of it. The PW120 is the same series engine as the PW118, and the 118 must be in feather for 20 seconds before shutdown not for cooling purposes, but to ensure that the prop aux feather oil reservoir is full. We observe 3 minutes with the condition levers in minimum after landing for engine cooling.

Quoting N231YE (Reply 19):I also believe this is a safety component built in

I was taught that the main reason you feather the free turbine PT-6 (KingAire) was to prevent them from continuing to turn after shutdown and in windy conditions. They sling out of feather when the eng. is started. There was no auto feather on any model KingAire I ever flew. We never went back into feather during taxi, etc. The PT-6 was such a dependable eng. that I only had to shut one down in the years I flew them and it was only precautionary due to a chip detector warning. The G-159 and YS-11 had auto feather for inflight eng. failure but in did not feather on shutdown...CC

Its what I was told, and it very well could be wrong (POH doesn't give a reason). In addition, after checking it is also actualy 30 seconds for us (embarrassing, but I haven't put a condition lever to fuel off since the sim almost a year ago). We also don't have any specified cooling off period after landing for shutting down #1. Heck, at outstations we've normaly got both shutdown and half the passengers off 3 minutes after landing.

Quoting Doug_Or (Reply 22):Its what I was told, and it very well could be wrong (POH doesn't give a reason).

Who told you? I've found that many line pilots don't know why we sit in feather for 20 seconds.

Quoting Doug_Or (Reply 22):Heck, at outstations we've normaly got both shutdown and half the passengers off 3 minutes after landing.

Yeah there are times when we sit at the gate for a minute waiting for the three minutes to come to a close, but the company determined (and I happen to agree with their assessment) that the reduction in engine wear is worth the very occasional extra minute before opening the door.

[Edited 2006-08-09 17:24:27]

09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0

25 Jetlagged
: 1200 to 1384 is still quite a narrow RPM range, which was my point, but I generalised too much regarding fixed settings. I did not know that the CT7

26 DarrenBe
: On the Saab at least, the prop rpm is not constant, when feathered. The rpm cycles between 500 and 950 rpm. Re cooling periods, the 3 minutes would c